1. Field of Invention
The invention relates to an electric wire for automobile. More particularly, it relates to an electric wire for automobile which meets the demand for an improved tensile strength and a smaller diameter.
2. Description of Related Art
An automobile uses a wire harness, which is a bundle of many electric wires, for electric connection with electrical equipment. Some of electric wires used in a wire harness include conductors having a twisted wire structure, which is obtained by twisting a plurality of element wires. FIG. 1 shows a typical conductor (element wire aggregate) included in this type of wire. In FIG. 1, denoted at 1 is the conductor having a twisted wire structure in which six peripheral element wires 3 are arranged around a single central element wire 2 in a single circle in tight adherence with each other and twisted. So far, in general, copper or copper alloy has been used as the central element wire 2 and the peripheral element wires 3, which form the conductor in such a twisted wire structure. Further, the diameters of the central element wire 2 and the peripheral element wires 3 are customarily the same. As a further general aspect, the nominal cross sectional area of the conductor is approximately 0.35 mm2.
Meanwhile, the recent years have seen an increasing demand to an electric wire for automobile for an improved tensile strength and a smaller diameter. However, in the case of the electric wire shown in FIG. 1, it is necessary to increase the diameter of the conductor to improve in tensile strength, which contradicts the demand for a smaller diameter.
In light of this, an object of the invention is to provide an electric wire for automobile which realizes a better tensile strength when the diameter of a conductor remains unchanged, maintains a tensile strength comparable to that of a conventional electric wire for automobile even when the diameter of the conductor is reduced, and achieves an equally favorable or better tensile strength than that of a conventional electric wire for automobile depending upon how thin the diameter of the conductor has been reduced.
As a result of intensive researches, it is possible to improve a tensile strength when stainless steel is used as a central element wire, that it is possible to realize diameter reduction while further improving the tensile strength when the diameter of the central element wire is made larger than the diameters of peripheral element wires. Further, when stainless steel, which exhibits a lower conductivity than copper or copper alloy, is used as the central element wire, the heat generation problem, i.e., combustion of the wire caused by the heat generated in the wire, is prevented as the amount of a fire retardant is set in a proper range.